U.S. patent number 6,735,535 [Application Number 09/724,159] was granted by the patent office on 2004-05-11 for power meter having an auto-calibration feature and data acquisition capabilities.
This patent grant is currently assigned to Electro Industries/Gauge Tech.. Invention is credited to Tibor Banhegyesi, Erran Kagan, Andrew Repka, Fred Slota.
United States Patent |
6,735,535 |
Kagan , et al. |
May 11, 2004 |
Power meter having an auto-calibration feature and data acquisition
capabilities
Abstract
A power meter is provided having an auto-calibration feature and
a data acquisition node for measuring the power usage and power
quality of electrical power in an electrical power distribution
network. The auto-calibration feature calibrates the power meter at
predetermined time increments and as a function of temperature
changes.
Inventors: |
Kagan; Erran (Port Washington,
NY), Repka; Andrew (West Islip, NY), Banhegyesi;
Tibor (Baldwin Harbor, NY), Slota; Fred (Coram, NY) |
Assignee: |
Electro Industries/Gauge Tech.
(Westbury, NY)
|
Family
ID: |
32233018 |
Appl.
No.: |
09/724,159 |
Filed: |
November 28, 2000 |
Current U.S.
Class: |
702/61; 324/142;
324/74; 702/60; 702/85; 702/86 |
Current CPC
Class: |
G01R
21/133 (20130101); G01R 35/04 (20130101); G01R
22/00 (20130101) |
Current International
Class: |
G01R
21/00 (20060101); G01R 21/133 (20060101); G01R
35/00 (20060101); G01R 35/04 (20060101); G01R
22/00 (20060101); G01R 021/00 (); G01R
035/04 () |
Field of
Search: |
;702/60,61,62,85,86,87,88,89,99,104,57,64,65,81,182
;324/74,141,142,140R,140D |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Nexus 1250 manual by Electro Industries/Gauge Tech. .
High Performance Switchboard Power Monitors manual by Electro
Industries/Gauge Tech..
|
Primary Examiner: Hoff; Marc S.
Assistant Examiner: West; Jeffrey R
Attorney, Agent or Firm: Dilworth & Barrese LLP
Parent Case Text
The present application claims priority to a United States
Provisional Application filed on May 5, 2000 by Kagan et al. having
U.S. Provisional Application No. 60/202,304; the contents of which
are incorporated herein by reference.
Claims
What is claimed is:
1. An auto-calibrating power meter comprising: circuitry configured
to receive a plurality of voltage and current inputs including
analog conditioning circuitry, which includes a plurality of
optical isolators for receiving the plurality of analog voltage
inputs and a plurality of current transformers for receiving the
plurality of current inputs, and sample and hold circuitry for
sampling and holding the plurality of voltage and current inputs to
be individually converted from analog to digital signals; and
processing circuitry configured to measure at least one parameter
of the received digitized plurality of voltage and current inputs,
to select at least one reference voltage during a calibration mode
for calibrating a range of measurements for the at least one
measured parameter of the received digitized plurality of voltage
and current inputs, and to compute at least one calibration factor,
which corrects drift errors occurring in the analog voltage inputs
from the optical isolators, for the range of measurements for
calibrating at least one parameter of the received digitized
plurality of voltage and current inputs; wherein the processing
circuitry executes several routines for computing the at least one
calibration factor in a hierarchical order of a 0.1 second routine
having a highest priority, a 1.0 second routine having a next
highest priority, and a main routine having a lowest priority.
2. The power meter according to claim 1, further comprising a metal
rod traversing through a toroid for connecting the plurality of
current inputs to a plurality of current outputs.
3. The power meter according to claim 2, wherein the toroid
contains approximately 1000 turns.
4. The power meter according to claim 2, wherein the toroid is
connected to a toroid sensor for measuring the current of at least
one current input of the plurality of current inputs.
5. The power meter according to claim 1, wherein the sample and
hold circuitry for receiving the plurality of voltage and current
inputs provides the inputs to a multiplexor for outputting one
input at a time to an analog-to-digital converter for digitizing
the plurality of voltage and current inputs.
6. The power meter according to claim 5, wherein the digitized
plurality of voltage and current inputs are provided to the
processing circuitry.
7. The power meter according to claim 1, further comprising voltage
reference circuitry configured to generate the at least one
reference voltage.
8. The power meter according to claim 1, further comprising a
calibration mode switch operatively controlled by the processing
circuitry for switching between a calibration position to compute
the at least one calibration factor and a non-calibration position
for receiving the plurality of voltage and current inputs.
9. The power meter according to claim 8, wherein the processing
circuitry switches the calibration mode switch to the calibration
position upon the periodic expiration of a timer.
10. The power meter according to claim 9, wherein the timer is set
to periodically expire every 15 minutes to every six hours.
11. The power meter according to claim 8, wherein the processing
circuitry switches the calibration mode switch to the calibration
position upon a temperature change of more than a predetermined
amount.
12. The power meter according to claim 1, wherein the at least one
parameter of the power meter that is calibrated is selected from
the group consisting of the gain and zero offset of the power
meter.
13. The power meter according to claim 1, further comprising
temperature sensor circuitry for sensing a temperature change.
14. The power meter according to claim 13, wherein upon sensing of
the temperature change by the temperature sensor circuitry, the
processing circuitry switches a voltage selection switch to select
the at least one reference voltage and a calibration mode switch to
a calibration position to compute the at least one calibration
factor.
15. The power meter according to claim 1, further comprising means
for communicating with a remote station.
16. A method for auto-calibrating at least one parameter of a
received plurality of voltage and current inputs in a power meter,
the method comprising the steps of: receiving the plurality of
analog voltage inputs and the current inputs by the power meter;
performing analog conditioning of the plurality of analog voltage
inputs and the current inputs utilizing a plurality of optical
isolators; sampling and holding the plurality of inputs to be
individually converted from analog to digital signals; measuring at
least one parameter of the digitized plurality of voltage and
current inputs; selecting at least one reference voltage for
calibrating a range of measurements for the at least one measured
parameter; and computing at least one calibration factor, which
corrects drift errors occurring in the analog voltage inputs from
the optical isolators, for the range of measurements for
calibrating the at least one parameter of the digitized plurality
of voltage and current inputs; wherein the at least one calibration
factor is computed in a hierarchical order of a 0.1 second routine
having a highest priority, a 1.0 second routine having a next
highest priority, and a main routine having a lowest priority.
17. The method according to claim 16, wherein the at least one
parameter of the power meter that is calibrated is selected from
the group consisting of the gain and zero offset of the power
meter.
18. The method according to claim 1, further comprising the step of
providing a communication link between the power meter and a remote
station.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical power distribution
networks. In particular, the present invention relates to a power
meter having an auto-calibration feature and a data acquisition
node for measuring the power usage and power quality of electrical
power in an electrical power distribution network.
2. Discussion of the Related Art
With the proliferation of electrically powered devices and systems,
there is an increasing need to accurately and precisely measure and
monitor the quality of the electrical power supplying these devices
and systems. While electrical utility companies currently use
devices to measure the amount of electrical power used by both
residential and commercial facilities and use devices to measure
the quality of electrical power in an electrical power distribution
network, these devices generally do not reproduce the incoming
electrical signal accurately in order to perform a detailed and
precise analysis of the incoming power quality and usage. Power
quality refers to the amount of power spikes, sags, surges, and
flicker, as well as other characteristics. As such, power quality
monitoring is especially important when expensive and sensitive
electrical equipment is connected to the power distribution
network.
Therefore, there exists a need in the art for a device which can
accurately and precisely reproduce an incoming electrical signal
and perform detailed and precise electrical power quality and power
usage analysis.
SUMMARY OF THE INVENTION
The present invention provides a power meter having an
auto-calibration feature and a data acquisition node for measuring
the power usage and power quality of electrical power in an
electrical power distribution network.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further explained by way of example and with
reference to the accompanying drawings, wherein:
FIG. 1A is a diagram of a power meter according to the present
invention;
FIG. 1B illustrates a coupling device of the power meter of FIG. 1
for coupling the power meter to an electrical power line to protect
the power meter from excessive current; and
FIG. 2 is a block diagram of an auto-calibration feature of the
power meter of FIG. 1A.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention combines the features of a high end revenue
meter with advanced power quality analysis capabilities. A feature
of the power meter of the present invention is its ability to
auto-calibrate voltage and current inputs, either on command or
upon the triggering of a certain event. The power meter of the
present invention periodically checks its temperature and
auto-calibrates when the temperature changes by more than a fixed
or predetermined amount, usually 1.5 degrees centigrade. The power
meter also auto-calibrates when a pre-set timer expires. The time
is preferably set to periodically expire every 15 minutes to every
six hours.
I. Hardware Operation
Referring to FIG. 1A, there is shown an exemplary diagram of a
power meter for calibrating voltage and current inputs according to
the principles of the present invention. The power meter is
designated generally by reference numeral 100 and includes a
plurality of voltage input lines 102, Vin, for receiving N voltage
inputs which are optically isolated by a respective optical
isolator 104, as known in the art, to protect the power meter 100.
The N voltage inputs are received from the respective optical
isolators 104 by a respective sample and hold circuitry 106 (S/H
1). Based on a control signal, the N voltage outputs of the
respective sample and hold circuitry 106 are transmitted to a first
multiplexor 108. The first multiplexor 108 receives a control
signal (SIGNAL 2) from a DSP 110 to output at least one of the N
voltage outputs received from the respective sample and hold
circuitry 106. The voltage output from the first multiplexor 108 is
received by a first analog-to-digital converter 114 which converts
the analog voltage output signal to a digital voltage signal. The
digital voltage signal is subsequently transmitted to the DSP
110.
The power meter 100 also includes a plurality of current input
lines 120, Iin, which receive N current inputs which are sampled
through respective current transformers CT. The N current inputs
are transmitted to a respective amplifier 122 and then to
respective sample and hold circuitry 124 (S/H 2). Based on control
signal (SIGNAL 1), the sample and hold circuitry 124 transmits the
N current inputs to the first multiplexor 108. Subsequently, based
on control signal (SIGNAL 2), the first multiplexor 108 outputs a
current output to the first analog-to-digital converter 114, which
is subsequently transmitted to the DSP 110.
The N voltage inputs and N current inputs are also received by a
second multiplexor 126 and at least one voltage input and at least
one current input are transmitted to a second analog-to-digital
converter 128 based on the control signal (SIGNAL 2). The outputs
from the second analog-to-digital converter 128 are transmitted to
the DSP 110. The DSP 110 is connected to peripherals, such as a
keyboard 130, a display 132, a modem 134, and a network card
interface 136 for communicating with the power meter 100 from a
remote station (not shown), preferably through a network
connection.
The digital representation of each of the N voltage and N current
inputs is processed and stored within the DSP 110. The DSP 110
includes at least a random access memory (RAM) and a read only
memory (ROM).
Referring to FIG. 1B, there is shown a coupling device for sampling
the N current inputs while protecting the power meter 100 from
excessive current. The input and output currents Iin, Iout are
connected via a U-shaped metal rod 200, which is preferably 1/4
inch thick, that bears current for the input current signal Iin.
The current of the input current signal In is measured via a toroid
sensor 202 attached to a toroid 204. The toroid 204 is implemented
to preferably convert the input current to a proportional voltage.
The U-shaped metal rod 200 traverses through the toroid 204 for the
input current signal Iin, as noted above. The metal rod 200 also
acts as a primary winding having a single turn and the toroid 204
acts as the secondary winding. The toroid 204 preferably contains
approximately 1000 turns.
FIG. 2 illustrates a simplified block diagram of the power meter
100 illustrated by FIG. 1A, including the components for
implementing the auto-calibration feature of the power meter 100
according to the present invention. The N voltage and N current
inputs, preferably, four voltage inputs and four current analog
inputs, are received by signal scaling circuitry 300 which scales
the input analog signals to measurable levels, as known in the art.
The analog signals are fed to analog signal conditioning circuitry
302, which includes optical isolators 104 for receiving the N
voltage inputs and current transformers CT for receiving the N
current inputs, as well as amplifier 122 for conditioning the
analog signals, as known in the art. The analog signals are fed to
circuitry 302 if a calibration mode switch 304 controlled by the
DSP 110 is switched to a non-calibration position (non-Cal).
After the analog input signals are conditioned by circuitry 302,
the conditioned analog signals are provided to sample and hold
circuitry 306 which combines sample and hold circuits 106, 124.
Circuitry 306 "freezes" the analog signals and provides the signals
to a multiplexor 307 (analogous to multiplexor 108) which outputs
one signal at a time to an analog-to-digital converter 308
(analogous to the first analog-to-digital converter 114). The
digitized signals are then provided to the DSP 110, in order for
the DSP 110 to obtain accurate power quantity difference
measurements and other measurements, such as a time delay, between
the digitized signals.
The power quantity measurements are prone to deviate from the
actual quantities due to temperature changes. That is, if the
temperature changes, there is an adverse effect on the analog
signal conditioning circuitry 302, especially the at least one
optical isolator 104 and the at least one current transformer CT as
known in the art, which causes errors, i.e., the power quantity
difference measurements of the voltage and current inputs deviate
from respective ideal parameters.
In order to calibrate for the errors presented due to temperature
changes, component aging, etc., periodically, e.g., every 15
minutes, every one hour, etc., the DSP 110 switches the power meter
100 to the calibration mode. The power meter 100 is switched to the
calibration mode by the DSP 110 providing a signal to calibration
mode switch 304 to switch to a calibration position (Cal); by
providing a signal to a power on/off switch 310 to switch to a
power-on position (power-on) to provide an operating voltage to
voltage reference circuitry 312 for generating high, medium and low
reference voltages as known in the art; and by providing a signal
to a reference voltage selector switch 314 to select reference
voltages outputted by the voltage reference circuitry 312. During
each calibration cycle, the DSP 110 uses each of the three
reference voltages to calibrate a range of measurements for various
parameters of the voltage and current inputs and subsequently,
computing a correction factor for each of the range of measurements
for adjusting the gain and zero offset of the power meter 100 to
adjust for temperature changes.
The selection between the high, medium and low reference voltages
is determined by the DSP 110, according to the extent of the errors
presented within the analog signal conditioning circuitry 302. The
DSP 110 is aware of the extent of the errors, e.g., the amount of
deviation of the power quantity measurements from the ideal power
quantity measurements, as the extent of the errors has been
determined and stored within the DSP 110 during factory
calibration.
Specifically, upon selecting each of the high, medium and low
reference voltages, the reference voltage is provided to the DSP
110, in order for the DSP 110 to compute the calibration factors
using software routines, as discussed in the next section. The
calibration factors are then used to calibrate the power meter 100.
The calibration factors could include, for example, calibration
factors for adjusting the phase delay of the analog signals or for
compensating for the deviation amount regarding the power quantity
measurements.
The DSP 110 also includes temperature sensor circuitry 316, as
known in the art, for sensing a temperature change. Upon detection
of a predetermined temperature change, the DSP 110 automatically
switches the power meter 100 to the calibration mode, by
transmitting at least one signal to the calibration mode, power
on/off, and reference voltage selector switches 304, 310, 314 to
switch the switches to the calibrate position, power-on position,
and to each of the reference voltages, respectively, in order to
perform calibration of the voltage and current inputs, as discussed
above.
Upon properly calibrating the power meter 100 due to a temperature
change or the expiration of the timer, the DSP 110 provides a
signal to the calibration mode switch 304 to switch to the
non-calibration position. The DSP 110 also provides the same signal
or another signal to the power on/off switch 310 to switch to a
power-off position (power-off). Accordingly, the power meter 100
resumes normal operation, i.e., the power meter 100 operates in the
non-calibration mode.
II. Software Operation
The power meter 100 incorporates several routines which are
programmed as a set of programmable instructions within the DSP
110, in order for the power meter 100 to achieve a high degree of
accuracy. A Main Operating routine runs continuously and contains a
Reference Calibration procedure for calibrating the voltage and
current inputs as described in the previous section, a Vector
Calibration procedure, a Phase Calibration procedure and a Fast
Fourier Transform (FFT) procedure. In addition, a Sampling
Interrupt routine runs while the Main Operating routine is running.
The Sampling Interrupt routine is responsible for collecting
samples of the incoming voltage and current inputs, calibrating the
samples and performing various power calculations on the calibrated
samples to measure at least the power quantity of the inputs.
The Main Operating routine which is programmed as a set of
programmable instructions within the DSP 110 and controlled by the
DSP 110 will now be described. A runcheck flag is set which
indicates whether the Main Operating routine has run recently; if
not, the power meter 100 resets. The Main Operating routine
coordinates data and data flow with the DSP 110. The DSP 110
contains operating parameters and system commands and preferably
stores the information in a flash memory for retrieval by other
devices.
The different routines used by the power meter 100 follow a
hierarchy. A 0.1 second routine, in which data is processed, is the
most important and will interrupt any process when it is executed.
Second in importance is the 1.0 second routine in which data is
processed. Lastly, the Main Operating routine is executed. In this
manner, whenever one of the more important routines is ready to be
executed, the current location, i.e., address, and data of the
current routine, i.e., the routine that is to be interrupted, is
stored in a stack, the important routine is executed, and the
routine then pops the address and data from the stack and continues
with the routine that was interrupted. For example, if the Main
Operating routine is being executed and a 0.1 second flag is set
indicating that there is enough data to run the 0.1 second routine,
the Main Operating routine is interrupted and the 0.1 second
routine is executed.
Following the Main Operating routine is a Reference Calibration
routine, Ref Cal. The Reference Calibration routine, while
important, does not have to be executed all the time. It can be
periodically scheduled. The Ref Cal routine is executed whenever
the current temperature changes by more than a fixed amount,
usually 1.5 degrees centigrade or a timer expires. Preferably, the
timer is set to periodically expire every 15 minutes to every six
hours. Since the electrical properties of certain internal testing
circuit elements, especially within the analog signal conditioning
circuitry 302, change with temperature, this feature ensures that
the power meter 100 will be as accurate as possible.
Continuing from the Main Operating routine, the procedure asks for
the current temperature. If the temperature difference is greater
than 1.5 degrees centigrade from the previous value, the procedure
stores the new temperature as the previous value and
auto-calibrates. The procedure then checks to see if an Mcal ST is
set. The first run through the procedure, sets the Mcal ST flag to
false.
If the temperature value difference is not greater than 1.5
degrees, the procedure determines whether the Ref Cal procedure was
run in the last interval, preferably, the last 15 minutes. This is
accomplished by checking the status of a pass flag. If the pass
flag is set to true, the procedure checks the status of the Mcal ST
flag. If the Mcal ST flag is false, the procedure checks the Vcal
Flag, which determines if the procedure should run the Vector
Calibration routine (see below).
The first run through the procedure, sets the Mcal ST flag to
false. This causes the procedure to check a Vcal_ch value. The
Vcal_ch value determines what reference input voltage is applied
for sampling. If it is zero, the procedure sets the input to the
power meter 100 to a preset Clow value, e.g., -2.5 volts, and sets
a Clow Pointer (PTR) to store the calculated low value. The
procedure then sets the Mcal ST flag, sets the Cal ST (calibration
start) flag and increments the Vcal_Ch. The procedure then
continues in the Main Operating routine and checks the Vcal flag.
In this manner, when the Main Operating routine returns to this
procedure, i.e., the processor is not busy with other functions,
the Mcal ST flag will be true and the Cal ST flag will be true.
Thus, a group of samples will be collected with a known input
signal applied, such as, e.g., a -2.5V (Clow) input signal. The DSP
110 calculates and stores A/D values for an incoming electrical
signal (such as voltage or current) on all input channels, and
after a set number of samples, e.g., 128 samples, the Cal ST flag
is then cleared and on the next pass through, the stored samples
are accumulated and divided by the number of samples. This averaged
value is then stored at the Cal PTR location (previously Clow), and
the Mcal ST flag is cleared. The next pass through the routine,
with the Vcal_Ch previously incremented, will move to the Vcal_Ch=1
routine where the same process is repeated for a mid-value of 0
volts. Finally, the routine is performed for the Vcal_Ch=2 where a
high value, e.g., 2.5V, is applied. Once the Vcal_Ch=2 routine is
performed, the DSP 110 then performs several calculations when
Vcal_Ch=2 is negative. The mid-value becomes a new zero offset and
is combined with an original gain factor for the electrical signal
being analyzed to yield a grand total gain factor. The grand total
gain factor is used to calibrate the voltage and current samples.
If, for example, the max input value to the A/D converter is 5V,
then 5/(A/Dhigh-A/Dlow) is equal to the gain.
The Vector Calibration routine is performed initially at the
factory during factory calibration and only in the field upon some
kind of operator intervention, such as transmitting a control
signal from a remote control station to the power meter 100 to
perform the Vector Calibration routine. The Vector Calibration
routine accounts for the phase error associated within the power
meter 100 itself. The power meter 100 preferably samples all
channels and stores them. If the Vcal Flag is set, then for a
period of say eight seconds in the one-second routine, the DSP 110
will record, for example, eight readings in the case of eight
channels and compute an average gain. The Vector Calibration
routine will then compute the difference between the factory gain
and what it determined from the eight samples. The difference will
be a new overall calibration factor which corrects for errors
associated with the electrical components of the power meter
100.
A Phase Calibration routine is performed after the Vector
Calibration routine. The Phase Calibration routine applies a fixed
factor phase correction determined in the factory and has an
initial value of 1. This corrects for errors, i.e., deviations from
the ideal values, associated with the current transformers CTs
within the conditioning circuitry 302. The correction is applied as
a phase calibration to the sampled signals.
Following the Phase Calibration routine in the Main Operating
routine, a Fast Fourier Transform (FFT) is performed to calculate a
number of harmonics, e.g., up to 128 harmonics, of the input signal
as known in the art. Once the FFT is completed, the procedure
returns to the beginning of the Main Operating routine.
Running continually in conjunction with the Main Operating routine
is a Sampling Interrupt routine. The Sampling Interrupt routine
gathers digital representations of the incoming voltage and current
inputs and applies the calibration factors derived from the various
calibration routines in order to create and store accurate
representations of the input signal. Assuming the input signal is
60 Hz, this indicates that there are 60 cycles per second. The
Sampling Interrupt routine is done on a cycle-by-cycle basis. After
each cycle of sample data, the routine checks to see if it has 0.1
seconds worth of data. If it does not, the Sampling Interrupt
routine runs another cycle.
As soon as enough data is stored for 0.1 seconds, the Sampling
Interrupt routine sets the 0.1 sec flag to true. When enough
samples are gathered to perform calculations, either at 0.1 second
or 1.0 second, the Sampling Interrupt routine and the Main
Operating routine are interrupted and a corresponding 0.1 sec or
1.0 sec procedure is performed. The 1.0 sec procedure is similar to
the 0.1 sec procedure described above, but performs additional
calculations on the samples. The 0.1 second reading is performed
using approximately 384, i.e., 64*6, individual samples, and
voltage, current and power are calculated for these samples. These
values are calibrated and used for high speed reporting updates and
are not revenue accurate values. The 1.0 second reading is
performed after approximately 4000 readings and is used for revenue
accurate readings.
To summarize the 1.0 second routine, the process starts off by
calculating all full scale power factors for V, I and power (W).
These are computed differently for different types of current
transformers CTs. After the Full Scale (FS) values are determined,
the 1.0 second routine checks if there is an input hookup. If so,
the current is zeroed and the 1.0 second routine determines if
Vab,Vbc,Vca are equal to zero. If the voltage is not equal to zero,
the Phase Calibration routine is performed. This is the only
calibration routine that is performed on the 1.0 second level and
not on the individual samples. The Phase Calibration routine is
initially performed at the factory and a fixed number is stored in
the memory of the power meter 100.
The Phase Calibration routine is performed as follows. A power
factor (PF) is calculated by dividing watts, i.e. V*I, by the VA,
i.e. the RMS value of V*I. This power factor is then used to
compute a phase correction based on the PF and I readings. The
correction value is used to correct the watts for the phase error
and since the watts were changed when the new PF was calculated,
the Var must also be recalculated since Var is equal to the square
root of Va.sup.2-W.sup.2. When the unit is initially calibrated at
the factory a fixed calibration value is calculated and stored in
the unit. For example, at the factory, a signal with a value of 60
degrees is input and the calibration correction is adjusted up or
down until the DSP 110 measures a correct reading. When the unit is
placed in the field, current and voltage samples are obtained and V
and I are used to determine watts, W=VI.
Since PF is equal to cosine of the angle between the voltage and
current, the PF is used to determine an initial phase angle for the
angle of the actual signal. How far off the initial phase angle
differs from, for example, a factory setting of 60 degrees is
determined and adjusted for accordingly. The adjustment is
performed by taking the difference of the two angles, i.e., PF
difference, performing a proportion gain calculation and based on
the ratio, which is not linear but a cosine relationship,
calculating a correction phase angle. This value is added to the
original measured angle and an inverse cosine function is used to
obtain the exact representation of the phase. For a sample
calculation, see the summary of examples below.
Referring to the Sampling Interrupt routine, if the 0.1 sec flag is
not set, the Sampling Interrupt routine checks to see if the Qflag
is set. If it is set, Var (reactive power) measurements (i.e.,
volt-amp reactive part) are performed every 0.1 sec routine, four
samples are preferably recorded to setup Q readings. The samples
are always synchronous. While the Sampling Interrupt routine can
lose several samples, a group of 64 samples is preferably always
used for each cycle. It does not matter where in the cycle the
cycle is started as long as it consists of one full cycle. Thus,
continuous sampling is performed even if a sample is missed. When
the counter for the Var routine reaches zero, the Qflag is cleared,
the 0.1 sec flag is set and the count is set to 64. The Sampling
Interrupt routine then exits back to the Main Operating
routine.
After the Qflag is checked, the Sampling Interrupt routine checks
for a FFT flag. If it is set, the power meter 100 performs the FFT
procedure to determine the signal harmonics. Finally, if the FFT
flag is not set, the Sampling Interrupt routine checks for a
calibration flag, CAL fl. If this is set, the Sampling Interrupt
routine proceeds to a Cal mod to Sample routine which coordinates
the activities of the DSP 110 and other processors of the power
meter 100 to prevent erroneous readings.
The power meter 100 of the present invention records incoming
waveforms based upon status changes and the occurrence of other
events, such as power spikes and surges. The power meter 100 is
capable of high speed waveform recording for later use in
troubleshooting electrical power-related problems. In addition, the
power meter 100 records harmonics of the incoming signal to the
127.sup.th order. Additionally, the power meter 100 performs a
flicker analysis for power quality and system stability
applications. The power meter 100 also records the sequence of
events using an IRIG-B satellite timing system (GPS) which is able
to synchronize a recording of network events to an absolute time
within one millisecond.
The power meter 100 of the present invention is expandable and can
read a plurality of current and voltage input signals
simultaneously. In addition, the power meter 100 is remotely
accessible from a central monitoring station. The data obtained by
the power meter 100 can be downloaded to a monitoring station for
further analysis using an open protocol.
The power meter 100 of the present invention includes several
features which prevent and protect the power meter 100 from harsh
substation transients. Specifically, voltage and current spikes
from the substations may damage ordinary power and revenue metering
devices. The present invention incorporates optically isolated
voltage inputs which provides superior protection from voltage
spikes. In addition, as described above, the power meter 100
includes a quarter-inch, U-shaped bolt or metal rod which bears the
current inputs in order to protect the power meter 100 from the
current inputs (see FIG. 1B). The U-shaped bolt acts as a direct
short and monitors the input current using an internal toroidal
sensor which converts the input current to a proportional
voltage.
The power meter 100 of the present invention preferably includes an
on-board Ethernet card and an on-board modem for communicating with
a remote station. In addition, the power meter 100 includes a
multi-port RS485 communication terminal. An example will now be
described with respect to the power meter 100 of the present
invention.
Example
Start: Describe 1. Calibration ("Cal.") Reference, Vector Cal.,
Phase Cal. are all set to 1 for all current and voltage ranges.
a) There is one calibration and offset factor for each voltage and
current input.
b) The sample gain correction factor (SGCF) is: SGCF=(Range vector
factor).times.(Calibration factor)
c) The SGCF is computed for each range (120V--volts), (0.25A, 0.5A,
1A, 2.5A, 5A--current) for all voltage and current inputs and each
sample is multiplied by this factor.
Start: Apply-Gain
This following sequence calibrates the reading regardless of
whether a unit is calibrated or not. The two factors applied to
each sample are:
SGCFO--overall sample gain correction factor
SOF--sample offset factor
These factors are derived from the following factors obtained
during calibration at the set points for the range:
SGCF.sub.120V, SGCF.sub.0.25A, SGCF.sub.0.5A, . . . ,
SGCF.sub.5A
How these factors, the calibration factors and offsets are obtained
and computed are described in the calibration sections below. 1.
Sampling--For each voltage/current channel sample:
A/Dcal=(A/Dvalue.times.SOF).times.SGCFO
The samples are now calibrated for gain. 2. Compute ##EQU1##
This is synchronized to the cycle so after 0.1 or 1.0 sec. of
cycles, this is the Calibrated RMS Value of Volts/Current. 3. A/D
full scale for volts=432.3 V
Current=52.23 A
RMS.sub.V/I =A/DRMS.times.432.3V/52.23 A 4. All other readings are
derived from these values. 5. At the 1.0 sec computation, adjust
the current SGCFO factor for the range proportionality as
follows:
a) Get the last current reading: for this example, use I=0.75A
b) Determine which band it is in: for this value it is between cal.
points 0.5A and 1A.
c) Proportion the gain: assume the SGCF.sub.0.5A factor is 1 and
SGCF.sub.1A =2.
d) Compute: 1) New SGCF.sub.1 =(I.sub.m -0.5)/(1-0.5) In this case
(0.75-0.5)/(1-0.5) (2-1)+1=1.5 2)
SGCFO=SCGF.sub.1.times.Calibration factor This is the new value
applied to all samples in the Sampling Interrupt routine.
Apply Phase
Each Phase--A, B, C has phase factors at the calibration points for
several ranges at phase angles of 60 deg. between V and I. These
are PHF.sub.0.5, PHF.sub.1, PHF.sub.2.5, PHF.sub.5, PHF.sub.10. 1)
These factors correct Watts only. 2) Every second the Watt value
and the Power Factor PF are computed, where
PF=Watts/V.sub.RMS *I.sub.RMS
Cos.sup.-1 (PF) is computed to determine the phase angle. 3) Assume
phase angle for current PF=30 deg., I=0.75A, PHF.sub.0.5 =1,
PHF.sub.1 =2.
a) First as above find current range between 0.5A and 1A
Proportion gain:
Gain=[(cos(30)-cos(60))/1] [(I.sub.RMS -I.sub.L)/(I.sub.H
-I.sub.L)] [(PHF.sub.H -PHF.sub.L)]+PHF.sub.L
For this example PHF.sub.H and PHF.sub.L are the 1A and 0.5A PH
factors due to the current of 0.75A.
Compute Gain=[((0.866-0.5)/1) (0.75-0.5)/(1-0.5) (2-1)+1]=1.18
Multiply Watts times the computed gain and recompute.
Final PF=New Watts/V.sub.A
Start--Do Ref Cal 1) Schedule a manual or auto reference
calibration. 2) In the Sampling Interrupt routine for each channel
do the following:
a) High turn on 2.5 V reference calibration Measure 128 A/D samples
and average Convert the average to volts. Assume results are Cal.
High=2.4 V 3) a) Next do low--turn on -2.5 V.
do same calculations as 2)a) and assume
Cal. Low=-2.35 V 4) a) Next do mid--turn on ground
Compute and average 128 Values
Assume Cal. Mid=0.6 V
b) Cal mid is the offset factor SOF applied to each sample in part
1) of "Start: Apply Gain" section above 5) Compute Calibration
factor=5/(Cal.sub.H -Cal.sub.L)
Cal factor here is 5/(2.4-(-2.35))=1.052
This is factor used to multiply all the SGCF range factors to get
the SGCFO factor (see "Start: Apply Gain" section, above).
Start--Do Vector Calibration 1) Vector calibrations are manually
done at the factory. A calibrated volt/current is input and the
unit is calibrated for that range value. These are the SGCF values
mentioned above. It is assumed a reference cal has been previously
done. 2) Calibrate for each Range, e.g., 1A--Set SGCF.sub.1A to
1.
a) Input 1A then press button
b) Unit will average eight 1.0 sec readings for the range selected
and compute the difference between what was input and measured
SGCF.sub.1A =1A/(8 Ave. Amp values)
This is the value used in the "Start: Describe" section above.
Get SGCFO. 3) Repeat for all ranges, volts and amps.
Start--Phase Calibration 1) Set power factor of 0.5
Set inputs to phase ranges with input currents of 0.5, 1, 2.5, 5,
10 amps, respectively. 2) For each range, e.g. 0.5A, check display
reading against what it should be, i.e., as computed.
Increment/decrement the phase correction factor until the watts
read correctly.
This is the factor (PHF) used to correct the phase in the "Apply
Phase" section above.
Although the illustrative embodiments of the present disclosure
have been described herein with reference to the accompanying
drawings, it is to be understood that the disclosure is not limited
to those precise embodiments, and that various other changes and
modifications may be affected therein by one skilled in the art.
That is, those skilled in the art will envision other modifications
within the scope and spirit of the claims appended hereto.
* * * * *